I've just done an equo update && equo world;many packages were upgraded, including ATI drivers (I have an ATI graphical card)...after a reboot, when I log into kde, the desktop is loaded but then the screen turns black with only the mouse cursor...

I haven't tried your solution, but I tried to remove all the kde files (.kde .kderc) and then log again; kde rebuilt all the files and now I have a working kde desktop (without the sabayon them), which works also with 3d effects (not perfectly, though, due to ati issues). So the problem was the old sabayon kde desktop, I assume...